1. Field of the Invention
This invention generally relates to pulleys. More particularly, the present invention relates to an over-running clutch pulley which has particular application within an engine accessory system including an automotive alternator.
2. Description of the Prior Art
During operation of an engine, a belt drive system is sometimes used to power and operate the various accessory devices including, but not limited to, an alternator which provides electrical power to the vehicle. While several type of belt drive systems are in use, the system which is currently in favor is known as a serpentine drive system. Serpentine drive systems generally include a drive pulley connected to the crankshaft of the automobile""s internal combustion (I.C.) engine and a ribbed belt trained about the drive pulley. The belt is also trained about one or more driven pulleys which are in turn connected to the input shafts of the various accessories. An automatic belt tensioner is also provided to maintain the tension of the belt within the proper range. Most driven pulleys are provided in a one-piece design. These pulleys have no over-running capabilities, meaning that the pulley is rigidly mounted to rotate with the accessory input shaft. When the input shaft of the accessory device is running at high speed (up to 22,000 rpm for an alternator), a significant amount of inertia is built up within the accessory device. As a result of the combined inertia and the lack of over-running capabilities, relative slipping between the pulley and the belt can occur if the belt decelerates too quickly. If significant slipping of the belt occurs, an audible squeal will be produced. Not only is a squealing belt annoying from an auditory standpoint, but it also undesirable from a mechanical standpoint since it produces undue wear on the belt itself.
In a typical driving situation, the accessory belt will experience instances of large deceleration, such as during in a 1-2 up shift during wide-open throttle acceleration. The situation is made even worse if the throttle is closed or xe2x80x9cbacked outxe2x80x9d immediately after the transmission has been shifted. In these situations, the belt decelerates very quickly while the pulley, with the high inertia from the accessory, keeps rotating very quickly in spite of the friction between the pulley and the belt. Other drawbacks and potential problems of present designs include: belt resonance, flutter and flap; long unsupported belt lengths; torque fluctuations from loading and unloading of accessories; and steering shutter.
In attempting to cure the slipping of the belt and its associated problems, various design proposals have been put forward. One proposed design includes the use of higher belt tensions. However, in these designs the belt tends to wear even more quickly. Additionally, the bearings associated with the input shaft of the accessory also have shown greater wear and a shortened useful life. Various other tensioner constructions have been proposed, but those tensioners have generally exhibited poor performance and were costly to implement.
Attempts to cure the belt slippage and squeal problem have also proceeded in another direction. While early driven pulleys were constructed in a one-piece design, newer pulley designs have been proposed where the driven pulley itself exhibits an xe2x80x9cover-runningxe2x80x9d capability. This allows the driven pulley to rotate relative to the input shaft of the accessory and therefore accommodate the inertia built up within the accessory.
U.S. Pat. No. 4,725,259 issued to Miyata discloses a construction where the driven pulley is mounted to the input shaft via a one-way clutch. The clutch only engages when the angular velocity of the pulley is accelerating. Otherwise, the clutch slips relative to the input shaft of the accessory. This design is intended to smooth out the recurring fluctuations of instantaneous velocity in the belt that is typical of an I.C. engine. The design smoothes out the corresponding recurrent instantaneous slipping of the belt relative to the driven pulley. No specific constructions for the Miyata one-way clutch are given in the disclosure of this patent.
U.S. Pat. Nos. 5,139,463 and 5,156,573, issued to Butzek et al., disclose alternator pulley constructions in which a coil spring is disposed in a space between a hub attached directly to the accessory input shaft and a pulley mounted for relative movement exteriorly of the hub.
In the ""463 patent, the two ends of the coil spring are respectively bent radially inward and radially outward so that one will engage the hub and the other will engage the pulley. In this patent, the spring is wound so that when a positive torque is applied from the belt to the pulley, the rotational movement of the pulley will be transferred to the input shaft of the accessory as a result of the spring xe2x80x9cwinding-upxe2x80x9d and the tangs engaging both the hub and pulley. Whenever negative torque is provided from the belt to the pulley, the spring enables the input shaft from the accessory and the hub to rotate relative to the pulley.
In the ""573 patent, the coil spring has one end bent radially outward. Additionally, the spring includes two sets of volutes, an intermediate set located between the other set and the bent end of the spring. The diameter of the hub and the inner diameter of the non-intermediate volutes are such that the volutes engage the hub when the pulley is being driven by the belt. When a negative torque is established between the alternator pulley and the input shaft, the volutes loosen with respect to the hub and allow slipping to occur. Importantly, the Butzek design of the intermediate volutes allows for a xe2x80x9cresilient rotational motionxe2x80x9d to ease the shock loading between the pulley and the hub but can cause fatigue problems in the spring.
U.S. Pat. No. 5,598,913 discloses a one way over-running clutch pulley in which a coil spring engages a composite cylindrical surface which is defined by both the sheave and hub. The spring is oriented so that torque is transferred from the sheave to the hub when the sheave is driving the hub. The spring allows slip to occur between these two components during deceleration of the drive belt""s recurring speed and torque fluctuations.
While the clutch pulleys of these patents may operate adequately in some respects, they have drawbacks in others. First, some prior art pulleys have experienced resonant vibration problems, especially during idle and sub-idle conditions. Another problem in over-running pulleys is hard lock-up. During engine idle, the large torque fluctuations which are characteristic of the internal combustion engine are smoothed out by an over-running pulley such as the pulley disclosed in the ""913 patent. During the brief periods of deceleration, the pulley over-runs. Because of its stiff nature in the drive direction, the hub would over (theoretically) rotate slower than the sheave""s input speed of the alternator and pulley (or sheave and hub) are equal. Such immediate and hard lock-up can cause its own noise and vibration. These designs are typically much larger than the standard solid pulleys and can result in serious packaging problems.
In view of the foregoing limitations and shortcomings of the prior art devices, as well as other disadvantages not specifically mentioned above, it should be apparent that there still exists a need for an improved one-way, over-running clutch pulley.
It is therefore a primary object of this invention to fulfill that need by providing a one-way over-running clutch pulley which overcomes the limitations and shortcomings of the prior art. Such a clutch pulley would find particular applicability with the accessory drive system of an automobile where it would allow for relative slip or compliancy between the pulley sheave and hub at the onset of lock-up. It is also an object of the present invention to provide a one-way, over-running clutch pulley which accommodates large decelerations of the belt so as to reduce or eliminate the belt squeal and wear that normally occur as a result of the belt slipping relative to the pulley and other problems that result from resonant vibrations of the belt.
Another object of this invention is to provide an over-running clutch pulley which utilizes centrifugal forces in a positive manner during operation so as to enhance the transfer of torque to the accessory input shaft.
Still another object of this invention is to provide a one-way, over-running clutch pulley incorporating an internal spring to provide for the limited slip in the clutch pulley.
A related object of this invention is to provide a one-way, over-running clutch pulley which exhibits enhanced heat dissipating capabilities.
Another object of the present invention is to provide an over-running clutch pulley of a lightweight construction and low cost, yet which is of a durable construction.
Briefly described, these and other objects of the invention are accomplished according to the present invention by providing what is hereinafter referred to as a compliant, one-way over-running clutch pulley that is intended for mounting to a rotational input shaft of a device, such as the alternator of an automobile. While the clutch pulley of the present invention is being specifically discussed in connection with automotive vehicles, it will be easily seen that the present invention has general applicability in a wide range of situations, particularly where it is desirable to provide compliance between a rotational input member and an output member to eliminate belt squeal and resonant vibrations.
The clutch pulley of the present invention is engaged by a driving member, such as a belt, which is rotatably being driven. The clutch pulley has several main components including an inner pulley hub, an outer pulley sheave, a bearing and a spring.
The pulley hub includes a first portion through which a mounting bore is defined. The bore is sized and shaped to receive the input shaft of the driven device, such as an alternator in a vehicle accessory system. The input shaft of the driven device is mounted to the pulley hub so that relative rotational movement between the pulley hub and the input shaft is prevented. Accordingly, rotation of the pulley hub will cause rotation of the input shaft of the driven device, e.g. the alternator. An axial extension off of the first portion of the pulley hub defines a first inner surface which is generally cylindrically configured and is generally coaxial with the bore of the hub and the shaft of the accessory.
The pulley sheave has a first portion which is adapted to engage the driving member which causes rotation of the pulley sheave. A radial flange extends off of the first portion of the pulley sheave and defines a second inner surface, also generally cylindrically configured. The second inner cylindrical surface has a diameter which is substantially the same as the diameter of the first inner cylindrical surface of the hub. The first and second inner cylindrical surfaces are coaxial with one another and adjacently located so that they cooperate to define a common or composite inner cylindrical surface. This composite inner cylindrical surface defines a spring receiving cavity. This cavity also allows for use of a retaining nut to hold the pulley to the input shaft of the driven device or can include a threaded fastener integral to the hub.
In an effort to decrease the overall weight of the pulley, in one embodiment of the present invention the sheave has a composite construction. More specifically, the majority of the sheave is made of injection molded plastic while a steel insert is overmolded into the plastic and provides a hard surface (for contact with the wrap spring), a lip to axially restrain the spring and another surface for receiving the dust cap or seal of the pulley.
The pulley sheave is mounted to the pulley hub by a bearing assembly which permits relative rotation between the two components . The bearing assembly is located between concentric portions of the pulley sheave and hub.
Received in the spring cavity mentioned above is a wrap or coil spring. The freestanding outer diameter of the spring is slightly greater than the diameter defined by the inner cylindrical surfaces. As a result, the spring is in an interference and frictional engagement with the steel insert portion of the composite inner cylindrical surface and retained by the radially outward contact normal force inherently exerted by the spring. The winding of the spring is oriented in a direction that compressively loads the spring whenever the pulley sheave is positively driven or accelerated relative to the pulley hub. This action tends to unwind the spring and would effectively increase its diameter if it were not restrained by the inner cylindrical surfaces of the sheave and hub. Conversely, the diameter of the spring effectively decreases when the pulley sheave is negatively driven or decelerated relative to the hub.
The effective increase in the diameter of the spring during compressive loading, positive driving and acceleration of the pulley sheave relative to the pulley hub causes the spring to exert an increased radially outward contact normal force on the inner cylindrical surfaces of the sheave and hub. The normal forces, which result in engagement of the spring with inner cylindrical surfaces, increase exponentially along the spring helix reaching a maximum at the midpoint. Because the normal forces determine the frictional forces and therefore the torque load which can be carried by the present clutch pulley, the initial normal force caused by the interference fit between the spring and inner cylindrical surfaces obviates the need for a tang on the spring if a sufficient number of spring volutes engage both portions of the composite inner cylindrical surface. With this increased normal force being applied to both the first and second inner cylindrical surfaces, these surfaces effectively become xe2x80x9clockedxe2x80x9d together by the spring and torque is transferred from the input member (belt) through the pulley sheave and hub to the shaft of the driven device. Depending on the amplitude and frequency of these recurring speed and torque fluctuations, a resonant condition may be set up in the drive belt, causing a flapping motion resulting in noise and excessive belt wear.
To eliminate the resonant condition, the present invention provides for a construction which allows a xe2x80x9csofterxe2x80x9d or more compliant coupling between the spring and the surfaces , and therefore between the sheave and the hub. In one embodiment of the invention, compliancy is accomplished by defining a recess area or scallop where the first and second inner cylindrical surfaces meet. During a non-locked condition, that portion of the spring adjacent to the recess will not be in contact with the inner cylindrical surface. As lock-up initiates, the spring must first expand into the recess, allowing relative rotation to occur between the sheave and hub and delaying or easing the harshness and immediacy of the lock-up.
In a second embodiment of the invention, the recess of the first embodiment is formed by tapered sections which bridge the first and second inner cylindrical surface.
In a third embodiment, the spring itself is instead formed with a raised or reduced cylindrical surfaces.
When a deceleration or negative driving is experienced between the pulley sheave and the pulley hub, such as during a wide open throttle 1-2 up shift, the shaft of the driven device will not immediately respond to the deceleration because of the inertia built up within the driven device and the pulley hub will over-run the pulley sheave. This causes the spring to unload and xe2x80x9cwind-upxe2x80x9d, effectively decreasing the diameter of the spring. Even a very slight reduction in the effective diameter of the spring results in a corresponding reduction in the normal force exerted by the spring on the two inner cylindrical surfaces which is sufficient to xe2x80x9cunlockxe2x80x9d the first and second inner cylindrical surfaces from each other and permitted relative rotation between the two surfaces. The pulley hub can therefore rotate under the inertia of the driven device via the shaft while the pulley sheave can rotate under the influence of the input member or belt.
Relative rotation in this manner reduces or eliminates relative slipping between the belt and the pulley and, importantly, associated squeal and premature belt wear problems.
A cap is fitted to the opening of the spring cavity. In addition to a radial face the cap includes an axial sleeve in close fit engagement with the inner diameter of the spring. As provided, the sleeve can be used to retain lubricant in the immediate vicinity of the spring and, in combination with the radial face of the cap, prevents entry of dirt, water, salt and other contaminants.